1. Field of the Invention
The present invention relates to an exhaust emission purification control device for an engine. More specifically, to an exhaust emission purification control device for an engine that estimates an accumulated oxygen amount in a catalyst using a counter. In addition, the present invention relates to an exhaust emission purification device for an engine that is preferred to be used in a vehicle (such as a motorcycle) with an internal combustion engine.
2. Description of Background Art
A method for estimating an accumulated oxygen amount in a catalyst using a counter is used in, for example, an exhaust emission purification device that performs air-fuel ratio control of an oxygen sensor in the downstream of the catalyst. During operation a vehicle with this type of exhaust emission purification device, when air is injected into an exhaust system by, for example, exhaust secondary air or so-called Air Injection (AI), then oxygen is accumulated in the catalyst. Immediately after this, the air-fuel ratio of the exhaust gas that passes through the catalyst is liable to be shifted to the lean side by the influence of the oxygen accumulated in the catalyst. Accordingly, in order to revert to the ordinary air-fuel ratio control, it is necessary to reduce the catalyst until the accumulated oxygen amount in the catalyst becomes an appropriate amount. More specifically, a reduction treatment is to perform fuel injection with an air-fuel ratio richer than stoichiometric (theoretical air-fuel ratio). This is a treatment that supplies unburnt gas to the catalyst so as to reduce the accumulated oxygen.
When the reduction treatment is performed, there is a need for terminating the reduction treatment as early as possible because it is easier to drive in accordance with requests of a rider in the ordinary air-fuel ratio control. In order to have an early termination of the reduction treatment, it is preferred to supply fuel as rich as possible. However, excessively rich fuel may cause unburnt gas remaining in the exhaust gas after the catalyst. Performing the rich injection corresponding to the accumulated oxygen amount in the catalyst is important to ensure compatibility between shortening the time of the reduction treatment and high emission performance. Accurately detecting or estimating the accumulated oxygen amount in the catalyst allows correspondingly adjusting a rich-injection correction coefficient during the reduction treatment. Methods for estimating the accumulated oxygen amount in the catalyst include, for example, a technology that estimates the accumulated oxygen amount using a counter while performing lean operation as disclosed in JP-A No. 2000-054826.
Meanwhile, assume that the AI, the reduction treatment, and a normal termination of the reduction treatment are always performed as one sequential set. It is only necessary to perform a count only with addition during performing the AI and then reset the counter to 0 at the normal termination of the reduction treatment. However, in the case where an operating range changes during the reduction treatment, for example, in the case where operations of opening the throttle after starting to move from the stop state, suddenly closing and then opening the throttle again, alternatively, operations of opening the throttle after a shift-up, suddenly closing and then opening the throttle again is performed, the reduction treatment after the first-time AI performance is forcibly terminated, that is, aborted in the middle, and then the second-time AI is performed. In this case, the accumulated oxygen amount at the time point when the first-time reduction is aborted is used as the initial value when the second-time AI is started. Thus, it is likely to improve the estimation accuracy of the accumulated oxygen amount.
As a means for estimating the accumulated oxygen amount at the point in time when the first-time reduction is aborted, JP-A No. 2000-054826 discloses a method for estimating the accumulated oxygen amount by subtraction of the count value during the rich injection in the middle of the reduction treatment.
However, with respect to a subtraction counter in the method of JP-A No. 2000-054826, the control needs to be established taking into consideration the operating state during the reduction treatment, environmental factors, degree of deterioration of the catalyst, and similar parameter. In the past, there is a problem with respect to setting of the control, setting a table for the subtraction counter, and similar settings that are laborious.
JP-A No. 2002-309928 discloses a known device as a conventional exhaust emission purification device for an engine. This device described in JP-A No. 2002-309928 is an air-fuel ratio control system where only one O2 sensor is disposed in a downstream of a catalyst, and increases the fuel injection amount until the output of the O2 sensor in the downstream of the catalyst (hereinafter referred to as SVO2) becomes a rich output during reduction of the catalyst. The control that performs a so-called rich spike is disclosed.
JP-A No. 2002-309928 does not disclose details. However, in the O2 feedback system in the downstream of the catalyst, when an accumulated oxygen amount of the catalyst increases and then a reduction treatment is performed, the air-fuel ratio in the downstream of the catalyst is shifted to the lean side due to the influence of the oxygen accumulated in the catalyst. This does not allow an ordinary O2 feedback control. Accordingly, the O2 feedback control is temporarily stopped. A fuel injection amount increasing correction on the catalytic reduction treatment is performed in an open-loop control until the SVO2 becomes a stoichiometric output (with the theoretical air-fuel ratio) or a rich output. When the SVO2 has become a stoichiometric output or a rich output, the control returns to the ordinary O2 feedback control.
When the reduction treatment is performed, an ordinary air-fuel ratio control is preferred for driving in accordance with requests of a rider. There is a need for terminating the reduction treatment as early as possible so as to revert to the ordinary fuel injection control. In order to early terminate the reduction treatment, it is effective to ensure an air-fuel ratio as rich as possible during the rich spike. However, if an excessively rich air-fuel ratio is provided for the oxidation capacity of the catalyst at that time point, unburnt gas is liable to be exhausted to the downstream of the catalyst. Accordingly, a margin to avoid an excessively rich air-fuel ratio is set during the rich spike so as not to output unburnt gas. A problem is that in order to have an early termination of the reduction treatment, this margin is required to be omitted to the extent possible.
Furthermore, when the control is switched to the O2 feedback control after the catalytic reduction treatment is terminated, there is a time lag until the output of the O2 sensor in the downstream of the catalyst responds. Here, a control input is calculated from a deviation between the SVO2 output at the time point of switching to the O2 feedback and the target SVO2 output of 500 mV, as the deviation of the O2 feedback. Accordingly, an excessive correction to the rich side may occur.